The heart is formed by a right and left side. Each side has a top (atrium) and bottom (ventricle) chamber. During a normal heartbeat, the blood flows from the body into the right atrium, and is then pushed into the right ventricle. The right ventricle contracts and pushes the blood into the lungs where it gathers more oxygen. From the lungs, the blood is pumped back into the left atrium of the heart, which then pumps it into the left ventricle. The left ventricle is the strongest part of the heart. It contracts strongly to pump blood to all areas of the body.
During each normal contraction, an electrical signal is also generated from an area at the top of the heart called the sinoatrial (SA) node. The signal then moves systematically through the atria and then down through the ventricles. This allows for a rhythmic, organized contraction and is called a normal sinus rhythm. During atrial fibrillation, however, electrical signals are generated from multiple sites in and around the atria, causing the atria to contract so rapidly they quiver, or fibrillate. As a result, the atria do not contract (squeeze) completely, reducing the amount of blood pumped into the ventricles. The ventricles do not fill completely and are then unable to pump enough blood to sufficiently supply the body’s other organs. There is also a small structure in the left atrium called the left atrial appendage which acts like a pocket in the quivering atrium. Since the atria do not contract completely during AF, a small amount of blood collects in this pocket and eventually can form a clot. The clot can then release into the blood stream and work its way to the brain. This is what causes a stroke in patients with AF.